The proposed research is intended to elucidate a variety of problems relating to the thyroid gland. The broad aim is to further basic knowledge regarding the thyroid gland and the action of its hormones, thyroxine and thiiodotyronine. Among the major topics to be investigated are: I. Improvements in isolation and purification of thyroid peroxidase, including human thyroid peroxidase. II. Source of hydrogen peroxide in the thyroid gland. III. Mechanism of thyroid peroxidase-catalyzed iodination and coupling. IV. Mechanism of action of antithyroid drugs V. Mechanisms of adaptation of rats to iodine deficiency. VI. Thyroid hormone function in larval and adult Amphibia.